Huygens’ construction of spherical wavefront:
i. Spherical wavefront is formed when source of light is at a finite distance from point of observation.
ii. Let S be the point source of light in air. PQR represents spherical wavefront at any instant. The wavefront PQR acts as a primary wave which is propagated through air.
iii. According to Huygens’ principle, all the points on PQR will act as secondary sources of light and send secondary wavelets with same velocity ‘c’ in air.
iv. To find out new wavefront at a later instant ‘t’, draw hemispheres with P, Q, R…. as centres and ‘ct’ as radius in the forward direction.
v. The surface tangential to all such hemispheres is an envelope at that instant ‘t’. Such a surface is passing through the points P1, Q1,R1…. on the hemispheres and touching all the hemispheres. This surface is the new wavefront at that instant ‘t’.
vi. SPN1, SQN2, SRN3 are the wave normals at P, Q, R respectively.
vii. These wave normals show the direction of propagation of spherical wavefront.
viii. The new wavefront P1Q1R1 is parallel to PQR at every instant.
Note: The intensity of secondary waves varies from maximum in forward direction to zero in backward direction. This indicates that secondary waves are effective only in forward direction.
